Phase diagram of the vortex state in an amorphous Re6Zr thin film exhibiting inverse melting
In Type II superconductors, the vortex lattice can exhibit "inverse melting," transitioning from a liquid to a crystalline solid as temperature increases. While recently observed via scanning tunneling microscopy in a 20 nm thick amorphous Re6Zr thin film, this work investigates the corresponding d.c. transport and low-frequency magnetic screening responses. By identifying distinct signatures of these transitions and integrating scanning tunneling spectroscopy imaging, we construct a comprehensive vortex-state phase diagram in the magnetic field-temperature parameter space. Furthermore, we demonstrate that inverse melting is thickness-dependent: a 5 nm film retains an inhomogeneous liquid state, while a 50 nm film maintains a crystalline solid structure except near the upper critical field.